The invention relates to an arrangement for modifying a main magnetic field generated in a magnetic resonance tomograph. The arrangement has a carrier structure.
Commercially available magnetic resonance tomographs (MR tomographs), and consequently also MR spectrometers, with an imaging function are often equipped with such an arrangement. It usually comprises three gradient coils and/or a specific number of shim coils wound onto a common cylindrical carrier structure.
The invention also relates to a method for producing an arrangement of the type described at the beginning. Such production methods are known.
MR tomography represents an imaging method that is afforded great importance for example in medical diagnostics for representing the structure and function of tissue and organs.
In order to align nuclear spins of an object being examined, an MR tomograph generally has a main magnet, with which a main magnetic field that is as static as possible and virtually homogeneous, often also referred to as a B0 field, can be generated in a z direction. Often used for this purpose is a superconducting main magnet, with which a field strength of for example between 0.5 T and 3 T can be generated.
A gradient system serves especially for spatial encoding, by which locational information can be obtained by way of the measuring signals recorded by an RF receiving coil. For this purpose, electrical currents are specifically generated by special current sources, known as gradient amplifiers, and are passed through primary coils of an arrangement described at the beginning. The strength and direction of the currents can in this case be set by means of a control unit. In the prior art, x, y and z coils generally form the primary coils. With such a system of x, y and z coils it is possible to generate a primary magnetic field, which modifies the main magnetic field and the z component of which varies linearly in the examination region. A field gradient of any desired orientation may in this case be generated by suitably setting the coil currents. This is a consequence of the superposition principle and the fact that with the x coil (y, z coil) a primary magnetic field with a field gradient in the x direction (y, z direction) can be generated.
A gradient system may also have a shielding unit (“shield”) with three secondary x, y and z coils, with which a secondary magnetic field can be generated, which outwardly shields the primary magnetic field generated by the primary x, y and z coils, in particular to reduce or prevent disadvantageous interactions with other components of the MR tomograph, in particular with the cryostat and components of the main magnet. It is required in this case to couple the shielding coils electrically with the primary coils. This usually takes place subsequently by means of soldering, whereby the primary coils are electrically connected to the corresponding secondary coils. However, such a type of production is laborious.
An MR tomograph is often equipped with a shimming system. The shimming system has the purpose of compensating for inhomogeneities of the main magnetic field, known as “shimming”. With the x, y and z coils of a gradient system, only linear inhomogeneities can be corrected. In the case of “active shimming”, the prior art provides for the correction of inhomogeneities of a higher order by using shim coils, which are designed to generate magnetic fields of a higher order, each shim coil generating a magnetic field of a specific order.
Since the requirements of shimming systems and gradient systems are different, the type of construction of gradient and shim coils may also differ. For example, there are different requirements for the switching speed and field strength that can be generated, so that the coils may have significantly different inductance and resistance values.
In the case of the customary MR tomographs with a cylindrical examination region, the coils of gradient and shimming systems are also respectively formed in a cylindrical manner and extend over the entire length of the examination region. The coils in this case form cylindrical layers with different radii and are mounted on a common carrier or form a common carrier structure, which is usually stabilized by casting, for example in epoxy resin.
In response to the only limited flexibility of such gradient and shimming systems with regard to the magnetic fields that can be generated, for some years efforts have taken a new direction. Systems known as matrix gradient systems have a multiplicity of relatively small individual coils, which in the case of closed systems are often arranged on a cylindrical surface and in the case of open systems are often arranged distributed on two planar surfaces. In this case, the multiplicity of individual coils in the case of the closed systems are arranged on a cylindrical carrier element and in the case of the open systems are arranged on at most two planar carrier elements that are spaced apart from one another and not neighboring one another.
Due to the fact that a high degree of flexibility can be achieved with matrix gradient systems with regard to the form of the magnetic fields that can be generated, such systems can also be designed as matrix shimming systems with suitable resistance and inductance values, or else such systems can be designed as combined gradient-shimming systems.
Due to the fact that the matrix technology is a new technological development and due to the complexity of matrix gradient systems, the few systems that are currently known still leave great room for improvement and are in great need of improvement.
DE 102 19 769 B3 discloses a magnetic resonance apparatus and a carrier device that can be equipped with shim elements, wherein a gradient coil system substantially has the form of a hollow cylinder, at least one receiving space extends in the direction of a principal axis of the hollow cylinder and has at least a longitudinal extent which is approximately that of the gradient coil system, wherein there is formed at least one carrier device, which can be introduced into the receiving space and can be equipped with shim elements, and at least one hollow body for conducting a cooling medium that cools the shim elements, which can be arranged in the receiving space, and which extends with at least a longitudinal extent which is approximately that of the gradient coil system along the direction of the principal axis.
DE 198 56 802 A1 discloses a gradient coil for magnetic resonance scanners with a primary coil produced on a central carrier and a secondary coil that surrounds the primary coil and serves for active magnet shielding, which are both embedded in a casting compound, wherein a segment cage, preferably forming the carrier of the secondary coil, arranged between the primary coil and the secondary coil and embedded in the casting compound, is formed from preferably axially continuous plastic profiles.
DE 197 22 211 A1 discloses a method for producing an actively shielded gradient coil arrangement for a magnetic resonance apparatus with a primary gradient coil set for generating gradient fields within an examination space and a secondary gradient coil set for shielding from stray fields, wherein the following steps are carried out: constructing all of the gradient coils belonging to the primary gradient coil set and secondary gradient coil set individually or in subassemblies with one or two gradient coils on a first part of a casting mold, completing the casting mold for casting, casting the gradient coils with a casting material and removing the gradient coil arrangement from the casting mold after curing of the casting material.